General Discussion of Aerosol-Based Printing
Direct Write printing, defined as maskless printing of discreet structures on a substrate in a one-step process offers many advantages to conventional printing technologies such as lithography and chemical and physical vapor deposition. Indeed, Direct Write processes such as aerosol-based printing are far less expensive to establish and maintain, and offer greater flexibility than conventional techniques. Embodiments of the present invention offer methods and apparatuses for aerosol-based direct printing of discreet structures using a multi-lens aerodynamic focusing assembly and a material shuttering assembly that produces shuttering of an aerosol stream within a print head. The apparatus of the invention produces discreet structures by shuttering a continuous stream of aerosol particles using a pneumatic shutter. In the most common embodiment, the Direct Write apparatus is comprised of a print module, a process vision module, a part alignment module, a shutter assembly, and a motion control module. The print module consists of an aerosolization source, a pressure and vacuum source, a cold plate, and a print head. The process vision module provides real-time viewing of the deposition process. The alignment module is used to define the vector distance between the axis of an alignment camera and one or more print heads, and for substrate alignment. The motion control module provides computer-controlled multi-axis motion of the substrate, and coordinated shuttering of the aerosol stream. The invention is capable of printing features as small as approximately 10 microns, at shuttering speeds as fast as approximately 10 milliseconds.
Aerodynamic Focusing Using an Aerodynamic Lens
The use of aerodynamic lenses to focus an aerosol stream was first reported by Lui. An aerodynamic lens can be defined as a flow configuration in which a stream traveling through a cylindrical channel with diameter D is passed through an orifice with diameter d, undergoing one contraction upstream of the orifice and one subsequent and immediate expansion downstream of the orifice. A contraction of an aerosol stream is produced as the flow approaches and passes through the orifice. The gas then undergoes an expansion as the flow propagates downstream into a wider cross sectional area. Flow through the orifice forces particles towards the flow axis, so that the aerosol stream is narrowed and collimated. Aerosol streams collimated by an aerodynamic lens system have been designed for use in many fields, including pharmaceutical aerosol delivery and additive manufacturing. In the typical aerodynamic lens system, an aerosol stream is tightly confined around the axis of a flow cell by passing the particle distribution through a series of axisymmetric contractions and expansions. Each section of the lens system consisting of a flow channel and an orifice is defined as a stage. Lui has presented a method and apparatus for focusing sub-micron particles using an aerodynamic lens system. Di Fonzo et. al. and Dong et. al. have designed lens systems that focused particles with diameters in the range from 10 to 100 nanometers and 10 to 200 nanometers, respectively. Wang has designed a lens system to focus particles in the range of 3 to 30 nanometers. Lee has reported a method of focusing micron-sized particles at atmospheric pressures using a single lens system composed of multiple stages.
In U.S. Pat. No. 6,348,687, Brockmann discloses an apparatus for generating a collimated aerosol beam of particles with diameters from 1 to 100 microns. The aerodynamic lens system of Brockmann uses a series of fixed lens and an annular sheath gas surrounding a particle-laden carrier gas. The system of Brockmann was used to focus 15-micron aluminum particles to a diameter of 800 microns, and generally uses the same aerosol and sheath gas flow rates. Lee (U.S. Pat. No. 7,652,247) discloses an aerodynamic lens system for focusing nanoparticles in air with diameters between 5 and 50 nanometers. In U.S. Pat. No. 8,119,977, Lee discloses a multi-stage, multi-orifice aerodynamic lens for focusing a range of particle diameters covering two orders of magnitude, from 30 to 3000 nanometers. In U.S. Pat. No. 6,924,004, Rao discloses a method and apparatus for depositing films and coatings from a nanoparticle stream focused using an aerodynamic lens system. The apparatus of Rao uses high-speed impaction to deposit nanoparticles on a substrate. A method of separating particles from a gas flow using successive expansions and compressions of the flow created by an aerodynamic lens is discussed by Novosselov in U.S. Pat. No. 8,561,486.
Aerodynamic Focusing for Direct Printing Applications
The general embodiment of the invention uses a method for stable direct printing of discreet structures on a substrate using aerodynamic focusing and pneumatic shuttering to produce highly collimated beams of sub-micron and micron-size droplets using an aerodynamic lens system, an annular sheath flow closely matched to the output of an aerosol source, and a combination compression and vacuum shuttering flow propagating perpendicular to the combined aerosol/sheath flow. In the preferred embodiment of the invention, the aerosolization source is a low-power ultrasonic atomizer operating in a continuous or pulsed mode. The atomizer described herein produces a relatively narrow range of droplet diameters, from approximately 0.5 to 5 microns, facilitating the production of a narrow, collimated aerosol beam. The atomizer power is typically less than approximately 10 watts. The lens system may consist of a single stage or multiple stages. The present invention is used to deposit well-defined traces onto various substrates with sub-micron edge definition. The apparatus uses interchangeable and variable aerodynamic lenses with configurations that can be tuned to match the aerosol output of the aerosol generator, so that a high degree of collimation of the aerosol beam is obtained.
In a Direct Printing technique, an ink is deposited onto a substrate without the use of masks or lithographic techniques. The present invention uses an aerodynamic lens system to form a thin aerosol jet surrounded by a sheath gas. The diameter of the core aerosol distribution is a function of the lens parameters such as channel length, lens orifice diameter, and the length of the lens. In the print mode, the apparatus propagates a combined sheath gas flow and an aerosol carrier gas flow through an aerodynamic lens system. The lens system is commonly terminated by a converging fluid dispense nozzle with an exit orifice positioned over a substrate. The distance between the exit orifice and the substrate is referred to as the working distance.
Internal Pneumatic Shuttering of an Aerosol Stream
The present invention provides methods and apparatuses for internal, contact or non-contact shuttering of an aerosol stream for the purpose of printing discreet structures on a surface. The invention discloses an apparatus for fast shuttering of an aerosol stream or a sheathed aerosol stream. Embodiments of the invention can be applied to, but not limited to processes requiring coordinated shuttering of a stream of particles, such as aerosol-based printing of discreet structures for Direct Write Electronics, for aerosol delivery applications, or for various 3D Printing applications. The aerosol stream may be composed of solid particles or liquid droplets.
A critical characteristic of the invention is that shuttering of the aerosol stream is accomplished without the use of wetted or impacted parts that come into contact with the particle stream after the particles exit the print head. Interrupting the particle stream on the exterior of the print head can lead to defocusing of the stream, particle scattering, material buildup on the shuttering mechanism, and unintended deposition of material during shuttering. The present invention overcomes issues associated with external mechanical or external pneumatic shuttering.
Shuttering time is defined as the time delay between initiation of a shutter signal (normally a low-voltage TTL pulse) and the enabling or disabling of stable aerosol delivery. Shuttering times of the invention are limited by the inherent delay associated with the actuation of electromechanical components of the shutter valve system. Shuttering times can be as small as 10 milliseconds.
Inhibiting Aerosol Flow within the Print Head
In one embodiment, the invention uses a single solenoid valve or multiple solenoid valves exterior to the print head to divert the aerosol stream from the internal print head flow axis and through a vacuum exhaust port. In another embodiment, a single valve or multiple valve system is used to interrupt the flow of aerosol carrier gas to the aerosol chamber. The valve system adds the diverted gas flow to the sheath gas flow. By diverting the aerosol gas flow and simultaneously adding the aerosol gas flow to the sheath gas flow, the pressure within the print head remains constant, while flow of aerosol from the aerosol chamber is stopped. The shuttering process is therefore internal to the print head, that is, no impaction or diversion of the aerosol stream occurs exterior to the print head. In the preferred embodiment, no internal moving parts or aerosol/shutter impact surfaces are used. In another configuration, the apparatus uses combined co-propagating compression and vacuum flows to shutter the aerosol stream. In the preferred embodiment of the invention, a single three-way universal solenoid valve is used to direct the aerosol gas flow to the aerosol chamber during printing. During shuttering, the aerosol gas is diverted and combined with the sheath gas, so that aerosol flow from the aerosol chamber is inhibited, and shuttering of the particle stream is accomplished.
Decrease Working Distance
The internal aspect of the shutter allows for a decreased working distance and precise deposition of aerosol droplets with a size distribution as wide as 2 to 4 microns. While the multi-lens configuration of the invention allows for working distances as great as 20 mm, the working distance can be less than 1 mm, allowing for focusing of small droplets in the aerosolized particle distribution. The non-impact aspect of the shutter eliminates accumulation of droplets on shutter components, allowing for extended unassisted operating times. The use of an internal, non-impact shutter allows for unassisted printing of discreet features with repetitive shuttering for a minimum of eight hours.